Pyrolysis in the tubes of a furnace or thermal reactor accounts for almost all of the ethylene produced today. The feedstock properties and the furnace operating conditions dictate the effluent composition. To produce the desired product the reaction is performed at high temperatures (approximately 750.degree. to 1100.degree. C.). For some designs, higher reaction yields occur using relatively short residence times (0.05 to 0.6 seconds) and relatively small diameter tubes and relatively low hydrocarbon partial pressures. During pyrolysis the hydrocarbons break down and liberate carbon in the form of soot or coke. Over time this carbon results in the process of carburization of the tube inner radius; wherein carbon enters the metal and combines with chrome. Carburization can increase the hardness of the tubes making them more brittle and more susceptible to stress fractures. This is accompanied by an increase in forces within the tube metal due to volume expansions resulting from chromium carbide formations.
During its operating lifetime a furnace is subject to multiple shutdowns and startups which impose cyclic thermal stresses on the furnace tubes. The severity of the operating conditions are such that the furnace tube material can withstand a finite number of cycles before experiencing a failure. These can be characterized as a localized stress fracture or as catastrophic failures. The occurrence of carburization reduces the number of cycles the furnace tube material can withstand.
The liberation of the carbon atoms during pyrolysis also results in coke formation which can deposit onto the inside of the reactor tubes and eventually cause fouling. This decreases furnace heat transfer and effectiveness. At some point it is necessary to shut down the furnace for decoking the furnace tubes. These shutdowns not only cause the thermal stresses noted above, but are also very costly and time consuming. For example, a single shutdown can cost the producer hundreds of thousands of dollars. Thus, it is even more desirable to minimize the furnace tube coking rate. Therefore, it would be desirable to utilize a furnace tube material that experiences minimal coking and minimal carburization.
The selection of furnace tube alloys is complicated by competing factors. Certain elements such as iron, cobalt, and nickel improve physical properties but are generally considered to accelerate or catalyze the coking reaction, so it is generally desired to avoid using these as components in the furnace tube alloy. Other elements, such as silicon are known to inhibit coking during pyrolysis conditions, but adversely affect the physical properties of the alloy. Silicon, for example, would require use in such high alloy proportions that the resulting alloy would be unsuitable for furnace use since that alloy would not be weldable.